I have my own concept of HNMs =)
They're High Notorious Monsters, same thing as saying High-level Notorious Monsters, therefore I use HNMs, but I see that you don't like it that much so I'm gonna call them...
Free-Roaming Dangerous On The Clock High-level Notorious Monsters.
FRDOTCHNMS
I know you meant no harm, but I had to write this =)